This invention relates generally to knocking sensors for producing an output electrical signal indicative of the intensity of knocking which occurs in an internal combustion engine, and more particularly, the present invention relates to improvement in structure of knocking sensors, and in the method for manufacturing the same.
In internal combustion engines of motor vehicles, for instance, undesirable knocking sometimes occurs, causing the durability of the engine to deteriorate, and resulting in more fuel consumption. Such knocking can be prevented by controlling the ignition timing as well as the air to fuel ratio of the mixture supplied to the engine. In order to control the engine in this way, it is necessary to accurately detect the state of knocking. A knocking sensor is a kind of a pickup which is attached to the engine casing, and is used to detect the waveform of the engine vibrations. The output signal of the knocking sensor is fed to a suitable circuit in which the peak frequency in the spectrum of the vibration frequencies will be detected. The peak frequency has a variation depending on the sort and size of the engine, and usually resides in a range between 6 and 9 KHz. Since knocking sensors are required to pick up knocking vibrations around the engine under a condition in which S/N of the objective vibrations is very poor, it is necessary that a sharp resonance frequency of the sensor equals the peak frequency of the engine vibrations. The sharpness of the resonance frequency of the conventional knocking sensors is usually set to a value between 10 and 100 because of an upper limit due to the transient characteristic, and because of a lower limit due to a simple cantilever beam structure of the vibrator.
The above-described customary knocking sensors are attached, in use, to the engine casing for picking up knocking vibrations. In order to attach the sensor to the engine, the sensor has a threaded projection which will be screwed in a screw hole of the engine. The sensor also has a nut-like periphery. so that this nut-like portion is gripped by a suitable tool, such as a torque wrench, to rotate and therefore, to screw the sensor. However, conventional knocking sensors comprise a shielding cable or code which is fixedly attached to a pickup including a transducer, of the sensor. For this reason, an impact wrench cannot be used for screwing a conventional knocking sensor. Namely, if a conventional knocking sensor were rotated by an impact wrench, the shielding code would be damaged due to high speed rotation. Furthermore, the shielding code is apt to coil around the shaft of the impact wrench, obstructing the installing operation.